To a solution of 20 (210 mg, 0.25 mmol, 1.00 eq) in anhydrous DCE (7 mL) was added 4-chlorobenzaldehyde (43 mg, 0.38 mmol, 1.50 eq) in anhydrous DCE (7 mL) and acetic acid (18 μL, 0.38 mmol, 1.50 eq).